Foldamers Stabilized by Adjacent Donoracceptor Aromatic Units

One way of enhancing the selectivity in aromatic-aromatic interactions is to polarize the aromatic systems and make one unit rich in electron density and the other deficient. With two different kinds of monomer units in a linear oligomer, different folding motifs may be obtained from the sequence of the monomer units [98]. This highly efficient strategy was pioneered by Iverson and colleagues and the class of foldamers is called aedamers (aromatic electron donor-acceptor oligomers) [99].

1,5-Dialkoxynaphthalene (13) and 1,4,5,8-naphthalenetetracarboxylic diimide (14) were employed as the donor and the acceptor units in aedamers. For the corresponding monomers, the donor-acceptor association constant (Ka) was nearly one and two orders of magnitude higher than those for the acceptor-acceptor and donor-donor complexes [100]. Due to different distributions of electron density, the donor and acceptor pair can also have better geometrical overlap, which contribute to stronger solvophobic interactions as well as stronger van der Waals interactions.

Scheme 3.7

The monomers were derivatized with an amino group on one end and a car-boxyl group on the other end. In this way, aedamers not only can be synthesized through standard solid-phase peptide chemistry, but also can incorporate natural amino acids for functionalization and solubility. FMOC chemistry was used to construct oligomers 15a with different chain lengths using L-aspartic acid as the linker (Fig. 3.3). The folded structure was supported by spectroscopic studies such as UV and 1H NMR spectroscopy. With an increase in the chain length, higher degree of hypochromism was observed, consistent with stacked p systems. Whereas the absorption spectrum of the shorter oligomer (n = 1) was similar to that of a 1:1 mixture of the monomeric donor and acceptor, those of the longer oligomers (n = 2, 3) differed increasingly with respect to intensity and shape. Additional evidence for the stacked aromatic rings came from NMR studies, which

Fig. 3.3 Schematic representation of aedamer-type foldamers. (Reprinted with permission from Ref 102. Copyright 2000, American Chemical Society, Washington, DC.)

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